Preliminary evaluation of a novel intraparenchymal capacitive intracranial pressure monitor

Laboratory investigation

Restricted access

Object

Intracranial pressure (ICP) monitors are currently based on fluid-filled, strain gauge, or fiberoptic technology. Capacitive sensors have minimal zero drift and energy requirements, allowing long-term implantation and telemetric interrogation; their application to neurosurgery has only occasionally been reported. The aim of this study was to undertake a preliminary in vitro and in vivo evaluation of a capacitive telemetric implantable ICP monitor.

Methods

Four devices were tested in air- and saline-filled pressure chambers; long-term capacitance-pressure curves were obtained. Devices implanted in a gel phantom and in a piglet were placed in a 3-T MR unit to evaluate MR compatibility. Four devices were implanted in a piglet neonatal hydrocephalus model; output was compared with ICP obtained through fluid-filled transduction and a strain-gauge ICP monitor.

Results

The capacitance-pressure relationship was constant over 4 weeks, suggesting minimal zero drift during this period. There were no temperature changes around the monitor. Signal loss at the sensor was minimal in both the phantom and the piglet. Over 114,000 measurements were obtained; the difference between mean capacitive ICP and fluid-transduced ICP was 1.8 ± 1.42 mm Hg. The correlation between ICP from the capacitive sensor and fluid-filled transducer (r = 0.97, p < 0.0001) or strain-gauge monitor (r = 0.99, p < 0.0001) was excellent. In vivo monitoring was restricted to 48 hours due to problems with robustness in the clinical environment.

Conclusions

This preliminary study demonstrates minimal long-term zero drift in vitro, good MR compatibility, and good correlation with other methods of ICP monitoring in vivo in the short term. Further long-term in vivo study is required.

Abbreviations used in this paper: ICP = intracranial pressure; PCB = printed circuit board; VAD = ventricular access device.

Article Information

Address correspondence to: Richard J. Edwards, F.R.C.S.(Neurosurg), M.D., Department of Neurosurgery, Frenchay Hospital, Bristol, BS16 1LE, England. email: richard.edwards@nbt.nhs.uk.

Please include this information when citing this paper: published online May 27, 2011; DOI: 10.3171/2011.4.JNS101920.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    Photograph showing principal components of the capacitive ICP monitor. The diameter of the tip is 3 mm; the total length of the device is 65 mm.

  • View in gallery

    A: Photograph showing the pressure measurement setup under general anesthesia—an endotracheal tube (a); 25-gauge needle (b) inserted into a VAD, connected to a fluid-filled pressure transducer; incision (c) used to implant a VAD and ICP monitoring device; incision (d) overlying the bur hole (yellow dot) and intraparenchymal insertion point of the sensor tip; and incision (e) overlying device lead connecting sensor with the flange. Asterisk indicates position of the device flange. B: Graph of time-ICP curves over a segment of a recorded epoch showing scattered data points from both systems. C: Graph showing mean (SEM) recorded ICP values over the same epoch. D: Bland-Altman plot demonstrating the 2 sets of data obtained over the same epoch, in which most values lie within the ± 2.5 mm Hg zone. s = second.

  • View in gallery

    Left: Graph demonstrating pressure-capacitance relationships for 4 devices. The lower blue curve represents an uncoated sensor. Note that the x axis denotes absolute pressure in bars. Right: Graph showing 4 superimposed pressure-capacitance curves for 1 of the 4 tested devices. Curves were obtained 1 week apart over a total testing period of 4 weeks, confirming a stable pressure-capacitance relationship over the period.

  • View in gallery

    Pressure (mm Hg)–capacitance (pF) graph over the pressure range relevant to clinical monitoring, after the subtraction of atmospheric pressure, demonstrating a linear relationship.

  • View in gallery

    Magnetic resonance evaluation. Agar gel phantom (A) with incorporated ICP monitor (B) imaged on T2 fast spin echo sequences, showing significant artifact around the flange with minor additional signal drop in the plane of the sensor tip (white arrow). Sagittal T2 fast spin echo sequences (C and D) of piglet cranium without and with an implanted device showing signal dropout around the flange (double white arrows) and to a lesser extent around the sensor (single white arrow).

  • View in gallery

    Left: Graph showing the correlation of the ICP means from each epoch obtained with the 2 systems (Pearson r = 0.97, p < 0.0001). Right: Bland-Altman plot for all the ICP means from each epoch showing the most values within the ± 3 mm Hg zone.

  • View in gallery

    Graphs showing a comparison of the Codman and capacitive ICP sensors over a rapid ICP rise (25 mm Hg in 2 seconds). Left: Although there is a tendency for the capacitive sensor to under-read, it responds closely to the rise. Right: Correlation of the 2 systems during the same ICP change (Pearson r = 0.99, p < 0.0001).

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